1. Field of the Invention
The present invention relates to a navigation system capable of displaying positions of mobile objects on a map by using a mobile communication system consisting of mobile objects and a key station.
2. Description of the Related Art
In recent years, data communication technology has been remarkably advanced. With the advancement, a new navigation system has been developed and proposed. With the assist of this navigation system, mobile objects, such as vehicles, vessels, and aircrafts, when traveling, can check their present positions and travels toward and exactly reach their destined places.
A GPS (Global Positioning System) has been known as a system to determine the present positions of mobile objects with a high precision. Reference may be had to U.S. Pat. No. 4,114,155 and the references cited therein, and to ELECTRONICS, issued by Ohm Ltd., December, 1984.
A plurality of artificial satellites are used so that at least four satellites may constantly be observed from the earth. Each mobile object carries a GPS receiver which is for receiving and demodulating a microwave carrying a meaningful signal from each satellite. Through the modulation, the receiver detects orbiting data of the satellite that is contained in the received microwave signal having frequency of about 1 to 2 GHz. The present position of each satellite is obtained by applying the detected orbit data to Kepler's equation and solving the equation. A distance from one mobile object on the ground and each satellite can be obtained by measuring a propagation time of the signal transmitted from each satellite. To work out the present position of the mobile object, the simultaneous equations are solved for the present positions of the satellites and the distances between of the mobile object and the respective satellites.
In the mobile navigation system as applicants of the present patent application know, the GPS receiver, a data base unit storing map data, an image processing unit, and a display unit are all assembled into each navigation apparatus carried on each mobile object.
The position data of a mobile object derived from the GPS receiver and the map data derived from the map data base unit are composed by the image processing unit. The image processing unit produces composite image data representative of an image in which a position of the mobile object is traced on a map. The display unit receives the image data and visually presents such an image on its display screen.
Such a mobile object navigation system is advantageous in that so long as the GPS operates, an operator, e.g., a driver, may always recognize the present position of the mobile object per se, but is disadvantageous in that the expensive map data base unit has to be mounted for each mobile object. Especially, when the mobile object travels in a wide area, since a tremendous amount of map data is required, a great volume data base unit has to be equipped for every mobile. Furthermore, since landform and roads change day by day, the map data must be frequently updated to guarantee exact travelings. Also, it is very difficult or impractical to frequently update the data base unit installed in the mobile object.